Infrared telescope spots mystery flare-ups in distant galaxies

SOME things that go bump in the night can only be seen with heat vision. SPRITEs, a new class of astronomical explosion, may be showing us never-before-seen phases in the lives and deaths of stars.

SPRITEs, short for “eSPecially Red Intermediate-luminosity Transient Events”, are undetectable in visible light. They were spotted only when Mansi Kasliwal at the California Institute of Technology in Pasadena and her team began monitoring 190 nearby galaxies with the infrared Spitzer space telescope in 2014.

In the first year, there were 14 odd flashes brighter than novae but fainter than supernovae. Strangely, these events were invisible to optical observatories like the Hubble or Keck telescopes.

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“The question is, what the heck did we just find?” Kasliwal says. There could be more than one answer to that.

The event we know the most about is called 14ajc. Its source appears to be a glowing cloud of warm molecular hydrogen, in the spiral galaxy Messier 83.

This might be what a star-forming nebula looks like after two young stars, each of about 10 solar masses, brush past each other or collide. That interaction would set off a shock wave through nearby interstellar gas, heating the nebula to produce the infrared glow (The Astrophysical Journal, doi.org/b56t).

But many of the SPRITEs have brightened and then faded more quickly than 14ajc. “I do believe that other events are not the same beast, because they just look different,” Kasliwal says.

Another possible source ofSPRITEs could be failed supernovae, says Stan Woosley of the University of California, Santa Cruz. In these events, a massive star collapses on to its hardened core, but there is no shock wave reverberating outwards and tearing the star apart, as in a supernova. Instead, the core crumples into a black hole and the explosion fizzles, leaving only a small outburst at the star’s outer layers.

It may take some time to differentiate between all the options, given that there are just a few observations with the Spitzer telescope. “They have found something interesting, but the available data is so sparse that it is difficult to come up with great hypotheses,” says Chris Kochanek at the Ohio State University in Columbus.

That’s something Kasliwal hopes to address. “How do we make this into an industry, from a cottage industry?” she says.

Since Kasliwal started looking in 2014, Spitzer has uncovered a total of 59 SPRITEs. Kasliwal has shared data about the most interesting ones online in real time, and is organising a workshop in September to study them further. She is also working on a new infrared detector to look for more. The James Webb Space Telescope should also help find answers when it launches in 2018.

“The observers are way ahead of us theorists,” Woosely says. “They are discovering things much faster than we can model or explain them.”

This article appeared in print under the headline “Infrared flare-ups reveal new events in stars’ lives”